1. Field of the Invention
The present invention relates to a limiting electric current type oxygen concentration sensor as one of oxygen concentration measuring sensors such as a magnetic type sensor utilizing the paramagnetic property of oxygen and zirconia type and galvanic cell type sensors which utilize the oxygen ionic conductivity.
2. Description of the Prior Art
Electrodes are formed to both major surfaces of an oxygen ionic conductive solid electrolyte plate, and an electric current flows through the solid electrolyte plate to move the oxygen for one direction. When a hermetic cover is provided for one of the electrodes, oxygen is supplied at a predetermined flow rate from an external source inside the cover, and a voltage is applied across two electrodes using the electrode at the side of the cover as a cathode, movement of oxygen ions can be effected in proportion to an amount of oxygen supplied thereto, so that an electric current flows from the anode to the cathode. The amount of oxygen flowing in the cover can then be calculated in accordance with the value of the electric current. The concentration of oxygen can be calculated if it is proportional to the flow rate thereof. The following sensor is conventionally proposed to serve as a limiting current type oxygen sensor which realizes the above-mentioned principle. Electrodes are formed to the upper and lower surfaces of a sintered solid electrolytic plate, and one of the electrodes is hermetically sealed with a cover. At the same time, an aperture is formed in a part of the cover such that the diffusion rate of oxygen in a gas to be measured and flowing therethrough serves as a determining factor such that the diffusion flow rate of oxygen is proportional to the concentration of oxygen. Alternatively, a porous coating layer is used in place of the cover with an aperture. In addition, electrodes are formed on inner and outer surfaces of a hollow cylindrical sintered body, and a coating is formed on an outer electrode such that the flow rate of oxygen gas is proportional to the concentration thereof. The sensor having the construction described above is heated by a heater to a temperature of 500.degree. to 800.degree. C. to get the oxygen gas ionized and conducted through the solid electrolytic plate. However, according to this conventional method, the oxygen concentration detecting device becomes large in size as a whole, and becomes complex in structure. Recently, a device having a sensor integrally formed with a heater has been proposed. However, the sensor of this type still presents the following drawbacks
(a) The sensor as a whole must be heated to a high temperature to realize good ionic conduction and hence decrease an electric resistance of the sensor.
(b) Variations in apertures of the cover and the porosity of the porous coating layer result in deviations in characteristics of the sensor.
(c) A lot of power is required to heat the sensor since the sensor is large in size.